Abstract: Considering the former and the recentprints of the 1/500.000 Scale Geological Maps of Turkey, anapproximately 40 km2 area between Seydiler, Ağlı town and İmranlar village located at the north ofKastamonu city in westhern Blacksea Region ispresented as orange colored Eoceneflysch in the former onewhile in the recent as unidentifıed Çuaternary. The same area is presented as Quaternary and Neogene inTunoğlu 1991a and 1991b. The unit spread out widely in this region is namedfor the first time as "Seydilerformation". The formation in question shows a paraconformable relation with Lutetian Gürleyikdereformation at the basement but an obvious hardground surface can be observed between them. At the top, it isoverlaid by recent alluviual deposits and observed as errosional surfaces. Litostratigraphical andpaleontological investigations were carried but on 184 samples collected from 7 measured stratigraphicsections that completes each other, whole rock analysis and related clay mineralogy and sandstonepetrography examinations were realisedon labratory research levelfor litostratigraphy and paleontologicalobservations were carried out particularly on planktic and benthic foraminifera, ostracoda, nannoplanktonand dinoflagellate assemblages. Considering all together the present microfauna, the age of Seydilerformation is assigned as Late Lutetian-Bartonian. In addition to environment determinating characteristicof the ostacod fauna, taking in consideration the other microfauna and flora present, besides lithologicalaspects and theirfacies characteristics, the deposition of Seydiler Formation is thought to be occured in amarine environment and at depths changing from neritic to bathial under transgressive conditions.